Intramolecular Charge Transfer and Locally Excited State Observation in Push–Pull 1,<i>n</i>-Bis(4-formylphenoxy)alkanes

Push-pull compounds including the 4-formylphenoxy group linked by saturated multimethylene bridges (CH₂) _<i>n</i>=2-4 forming bis-aldehydes were designed and synthesized. The effects of the flexibility and electronic nature of the multimethylene bridges on the ¹H NMR and UV spectroscopic characteristics and the fluorescence ability of these bis-aldehydes were investigated in different solvents. The results indicated that with an increase in the length of the multimethylene bridge, the inductive effect is enhanced, and a bathochromic shift and hyperchromic effect in their UV spectra were observed. The locally excited (LE) and intramolecular charge transfer (ICT) emission levels were observed in their fluorescence spectra, but the emission resulting from possible excimer formation was not observed. Additionally, the fluorescence emissions from the LE and ICT levels are intensified in the nonpolar and polar solvents, respectively. Density functional theory (DFT) computational results indicated that most of the UV transitions are of the π-π* nature and from the bridge side of each aryl ring toward the attached chromophore of the same aryl ring.